This invention relates to optical networks, and in particular, to flexible optical network architecture and add/drop multiplexer therefor, which provide flexible connection between the network nodes.
Optical Packet Networks (OPN) typically have Synchronous Optical Network (SONET) ring architecture with DWDM techniques integrated inside the rings to increase the network capacity and to support multi-ring connections and the multiple services requests. Although the OPN management can control the traffic flow through the network, the ability to deliver bandwidth on demand anywhere in the network is not always possible. Over-provisioning of the network can partially solve this problem, but only at the expense of the increase of the network costs and less effective utilization of the network resources. Additionally, the update and re-provisioning of the equipment has to be done quickly and almost in real time, which is definitely not the reality of today.
Optical communications systems have been employing different network architectures to provide required flexible connections between the network nodes and bandwidth on demand services. For example, in a fixed wavelength network, where each node transmits and receives channels at fixed wavelengths, the transmitted/received wavelengths are the same for those nodes that communicate with each other. This network architecture requires multiple transmitters and receivers at each node, or otherwise it does not have flexibility to provide multiple connections between different nodes. It is also costly and inefficient to upgrade such a network, e.g. to accommodate new channels or to establish new connections, as it will require the addition of extra transmitters/receivers at the nodes. As a result, with this network architecture, it is difficult to satisfy the ever-increasing demand for network growth.
To overcome the limitations of fixed wavelength networks, it has been suggested to use tunable wavelength transmitters and/or receivers to provide higher flexibility of the network connections. For example, in a Fixed-tuned Transmitter and Tunable Receiver (FTTR) approach, each node is assigned with a specific wavelength for data transmission, while a receiver is a tunable device capable of receiving one of several data streams at different wavelengths generated by the transmitters. To transmit data from node j to node i, signalling messages have to be first sent to inform node i to tune its receiver to wavelength xcexj for data reception. FTTR network architecture has been deployed, e.g. in a European experimental system named Rainbow-II networks and published in an article by Eric Hall et al. entitled xe2x80x9cThe Rainbow-II Gigabit Optical Networksxe2x80x9d, IEEE Journal of Selected Areas in Communications, Volume 14, No. 5, June 1996, p.814-823.
Another approach, where tunable devices are used at network nodes, is known as Tunable Transmitter and Fix-Tuned Receiver (TTFR) network architecture. In the TTFR approach, each node is assigned with a fixed wavelength for data reception, where the receivers at node i are only responding to the wavelength channel i (xcexi). Nodes intending to send data to node i have to tune their transmitters to wavelength xcexi. TTFR architecture has been described, e.g. in the article to Chun-Kit Chan et al. entitled xe2x80x9cNode Architecture and Protocol of a Packet Switched Dense WDMA metropolitan Area Networkxe2x80x9d, Journal of Lightwave Technology, Vol. 17, No. 11, November 1999, pp. 2208-2218, where TTFR concept has been applied to DWDM networks.
The major drawback of tunable devices is their high cost and low reliability compared to the fixed wavelength devices. Additionally, the process of wavelength tuning has finite response time, it is sensitive to temperature and/or current changes and therefore requires stabilization.
Thus, network architecture using fixed wavelength devices can provide quick and reliable connections, but fail to provide flexibility and cost effective solutions to accommodate network growth and utilization. In contrast, known network architectures using tunable devices can provide flexibility of network connections, but tend to be expensive, less reliable and more complicated in exploitation and maintenance.
Accordingly, there is a need in industry for the development of an alternative optical network and node architecture, which would deliver inexpensive, flexible and reliable network connections.
Therefore there is an object of the invention to provide an optical network architecture which would provide flexibility of the network connections while being simple and cost effective.
According to one aspect of the invention there is provided an optical network, comprising:
a plurality of N nodes;
each node has a transmitter for transmitting a set of xe2x80x9cn1xe2x80x9d wavelengths (transmitter set), and a receiver for receiving a set of xe2x80x9cn2xe2x80x9d wavelengths (receiver set), the transmitter and receiver sets are misarranged so as to differ by at least one wavelength; and
the wavelengths of transmitters and receivers at different nodes are arranged so that for any pair of nodes there is at least one common wavelength which is the same for the transmitter at one node and the receiver at the other node, thus providing a direct connection between the nodes.
Beneficially, it is arranged that for any pair of nodes there are at least two common wavelengths, the first and second common wavelengths, the first wavelength being the same for the transmitter at one node and the corresponding receiver at the other node in the pair, and the second wavelength being the same for the receiver at one node and the corresponding transmitter at the other node in the pair.
Conveniently, the total number of the wavelengths used in the network is equal to xe2x80x9cn1+n2xe2x80x9d, and the total number of nodes is equal to N=(n1+n2)!/(n1!n2!).
The transmitter set and receiver set may have different number of wavelengths, i.e. n1xe2x89xa0n2, and some or all of the wavelengths of the transmitter set may differ from the wavelengths of the receiver set.
Alternatively, the transmitter set and receiver set may have same number of wavelengths, i.e. n1=n2=n, and some or all of the wavelengths of the transmitter set differ from the wavelengths the receiver set.
The number xe2x80x9cn1xe2x80x9d of the wavelengths in the transmitter set may be the same for all nodes, or alternatively the number xe2x80x9cn1xe2x80x9d of wavelengths in the transmitter set may vary for different nodes.
According to another aspect of the invention there is provided an optical network, comprising:
a plurality of nodes, each node having a transmitter for transmitting a set of xe2x80x9cnxe2x80x9d wavelengths, and a receiver for receiving another set of xe2x80x9cnxe2x80x9d wavelengths, the set of wavelengths of the transmitter being different from the set of wavelengths of the receiver;
wavelengths of transmitters and receivers at different nodes being arranged so that for any pair of nodes there is at least one common wavelength which is the same for one of the transmitter and receiver at one node and one of the respective receiver and transmitter at the other node, thereby providing a uni-directional, direct connection between the nodes.
Conveniently, wavelengths of transmitters and receivers at different nodes can be arranged so that for any pair of nodes in the network there are at least two common wavelengths, the first and second common wavelengths, the first wavelength is the same for the transmitter at one node and the corresponding receiver at the other node in the pair, and the second wavelength is the same for the receiver at one node and the corresponding transmitter at the other node in the pair, thereby providing a bi-directional direct connection between the nodes.
Conveniently, the total number of the wavelengths used in the network is equal to xe2x80x9c2nxe2x80x9d, the total number of nodes is equal to N=(2n)!/(n!n!), and the number of common wavelengths for any pair of nodes is not exceeding xe2x80x9cnxe2x88x921xe2x80x9d.
The number of wavelengths used by transmitters or receivers at each node may be conveniently equal to n=2,3, 4 to 10, or any other number of wavelengths, which would provide required connection between the nodes in the network.
Preferably, transmitters and receivers at the network nodes are fixed wavelength devices, which generate or receive signals at fixed wavelengths. Alternatively, some or all of the transmitters and/or receivers may be tunable or switchable wavelength devices, which allow tuning or switching of the wavelength within a required wavelength range.
The network architecture described above can be applied to various types of optical networks, e.g. a wavelength division multiplexing (WDM) network, including ring, multi-ring, mesh, bus and star network topologies.
According to another aspect of the invention there is provided a node for an optical network, comprising a transmitter for transmitting a set of xe2x80x9cn1xe2x80x9d wavelengths, and a receiver for receiving another set of xe2x80x9cn2xe2x80x9d wavelengths, the set of wavelengths of the transmitter differing from the set of wavelengths of the receiver by at least one wavelength;
the wavelengths of the transmitter and receiver at the said node are arranged so that for any pair of nodes in the network, where the said node is one of the two nodes in the pair, there is at least one common wavelength which is the same for one of the transmitter and receiver at the said node and one of the respective receiver and transmitter at the other node in the pair.
Conveniently, the wavelengths of the transmitter and receiver at the node are arranged so that for any pair of nodes in the network, where the said node is one of the two nodes in the pair, there are at least two common wavelengths, the first and second common wavelengths, the first wavelength is the same for the transmitter at the said node and the corresponding receiver at the other node in the pair, and the second wavelength is the same for the receiver at the said node and the corresponding transmitter at the other node in the pair.
Conveniently, n1=n2=n, and the set of wavelengths of the transmitter is different from the set of wavelengths of the receiver.
Advantageously, the node described above further comprises an optical add/drop multiplexer/demultiplexer (OADM) including means for dropping wavelengths from the network at the node and means for adding wavelengths to the network from the node. Conveniently, the means for dropping wavelengths includes means for dropping one wavelength at a time, comprising a set of xe2x80x9cn2xe2x80x9d optical filters adjusted to the wavelengths of the receiver, and the means for adding wavelengths includes means for adding one wavelength at a time, comprising another set of xe2x80x9cn1xe2x80x9d optical filters or optical couplers suitable to operate at the wavelengths of the transmitter. Optionally, a node may further comprise a wavelength converter to provide indirect connection between the nodes through other nodes in the network.
According to yet another aspect of the invention there is provided an optical add/drop multiplexer/demultiplexer for a node described above, comprising:
means for dropping wavelengths from the network to the node comprising a set of xe2x80x9cn2xe2x80x9d optical filters suitable for operation at the wavelengths of the receiver set; and
means for adding wavelengths to the network from the node comprising one of the following:
a set xe2x80x9cn1xe2x80x9d optical filters suitable for operation at the wavelengths of the transmitter set; and
a set of xe2x80x9cn1xe2x80x9d optical couplers suitable for operation at the wavelengths of the transmitter set.
According to yet another aspect of the invention there is provided a method of providing direct connections between the nodes in an optical network having a plurality of nodes, the method comprising the steps of:
for each node, providing a transmitter for transmitting a set of xe2x80x9cn1xe2x80x9d wavelengths, and a receiver for receiving another set of xe2x80x9cn2xe2x80x9d wavelengths, and selecting the wavelengths of the transmitter and the receiver so as to differ by at least one wavelength;
selecting wavelengths of transmitters and receivers at different nodes so that for any pair of nodes there is at least one common wavelength which is the same for one of the transmitter and the receiver at one node, and one of the respective receiver and transmitter at the other node, thus providing a direct connection between the nodes.
Beneficially, it is selected so that n1=n2=n, the wavelengths of the transmitter set is different from the wavelengths of the receiver set. Advantageously, the step of selecting the wavelengths of transmitters and receivers at different nodes is performed so as to provide that for any pair of nodes there are at least two common wavelengths, the first and second common wavelengths, the first wavelength is the same for the transmitter at one node and the corresponding receiver at the other node in the pair, and the second wavelength is the same for the receiver at one node and the corresponding transmitter at the other node in the pair. Conveniently, the method described above further comprises the step of arranging the total number of wavelengths used in the network to be equal to xe2x80x9c2nxe2x80x9d.
Conveniently, the optical network described above, can be built as an overlay of a multi-ring network so that the nodes of the optical network are the nodes in the multi-ring network, thereby providing a flexible connection between the nodes in the multi-ring network.
The network architecture described above has the following advantages. It requires less wavelength resources to support the same number of nodes in the network than other known solutions, and it is more cost effective and reliable.